(USA TODAY) -- Tuesday's arraignment of James Holmes, accused of mass murder in Colorado, is a rarity among massacre suspects.

Holmes, scheduled to enter a plea for the July 2012 murders of 12 patrons in a suburban Denver movie theater and the attempted murder of nearly 60 others, is just one of a handful of suspects to face legal proceedings in a case involving no prior connection to murder victims.

Since 2006, there have been 200 mass killings - defined by the FBI as four or more victims. But although random shootings like those involving Holmes and Newtown, Conn., shooter Adam Lanza may be high-profile for the senseless nature of the killings and the random targeting of victims, there have been just 29 such massacres since 2006.

Most of the rest involved relatives and acquaintances, robberies, burglaries or disputes over illegal drugs.
Of the 29 massacre suspects like Holmes, few make it to trial. Fifteen committed suicide at or near the scene of the crime; three others were killed by police. Just five of the 29 - including Arizona shooter Jared Loughner - were ultimately convicted. Many were determined to be suffering from mental illness when they committed their crimes.

Daniel Ignacio is now on trial for the arson-related deaths of five Guatemalan immigrants in a Brooklyn, N.Y., apartment building. Awaiting trial: Army Maj. Nidal Hasan, a military psychiatrist accused of killing 13 and wounding 32 at Fort Hood, Texas, in November 2009.

In most cases, a motive will never be known. Although schizophrenia, delusions, depression and other behavioral issues factor into many mass public killings, some suspects' behavior before killing rampages don't raise sufficient red flags among family, friends, fellow workers or schools.

"In the vast majority of these cases, these are people who never have killed before," notes University of Alabama criminologist Adam Lankford, author of The Myth of Martyrdom: What Really Drives Suicide Bombers, Rampage Shooters, and Other Self-Destructive Killers.

Random mass killings appear to provide instigators both compensation for failures in their lives and a way to act out, Lankford says.

"These people feel they're victims of conspiracies or persecution. In that moment (of a killing rampage), they have more power and status than they ever had before. People are literally bowing down to them, ducking the bullets," he says.

Two murder cases ended with the suspects declared mentally unfit to stand trial. Two others are pending. In California, One Goh, charged with killing seven people at Oikos University, is undergoing evaluation every six months. Scott Dekraai, charged with eight murders in Seal Beach, is being treated for bipolar disorder but has not been declared unfit for trial.

Of the five people convicted in mass public slayings since 2006, four - including Loughner - had mental health diagnoses that did not prevent them standing trial. Just one - Conner Schierman, now on death row in Washington state for killing four neighbors in 2006 - did not have a mental health diagnosis. Schierman has apologized to victims' families, but gave no motive.

Holmes is expected to plead not guilty by reason of insanity, a move that would require an evaluation by state psychiatrists to determine his competency.

Holmes' attorneys have said in court hearings that the 25-year-old suffers from mental illness and had been treated by a University of Colorado psychiatrist before dropping out of the school's doctoral neuroscience program after failing a May final exam.

A Monday court ruling by presiding Judge William Sylvester clarifies Holmes' options. Should Holmes use an insanity defense, Sylvester said that Holmes could be medicated by state psychiatrists to determine his competency and that he could also be given a polygraph examination as part of the state's evaluation to determine whether he was legally insane at the time of the July 20 shootings.

Veteran criminal attorneys say Holmes has few options, noting the overwhelming evidence presented by prosecutors at a January preliminary hearing in which they showed Holmes' actions leading up to the shootings, including weapons purchases and staking out the Aurora theater complex, where he was arrested by Aurora Police.

"There is no defense other than insanity,'' says Craig Silverman, a former chief deputy Denver prosecutor. "There's a significant chance he's suffering from a mental disease or defect that rendered him incapable of knowing right from wrong."

If Holmes is judged capable of standing trial, he could ultimately face the death penalty if convicted of murder. Arapahoe County prosecutors have not said whether they are seeking capital punishment.

Even so, Colorado has not carried out an execution since 1997. The appellate process could drag out a case for years. Silverman notes that convicted Aurora, Colo., killer Nathan Dunlap has been on death row since 1993.

Dan Recht, a former Colorado state public defender, agrees with an insanity plea.

"This is not a 'Who Done It?' case,'' Recht says. "The evidence is beyond dispute. The only real question is whether his illness rises to the level of insanity and if he understood right from wrong when he committed those acts."

Recht says Holmes' case is comparable to that of Loughner, who suffered from mental illness but was put on medication then reached a plea deal that sent him to prison for life without parole.

A similar plea bargain in Holmes' case would avoid a costly trial and prevent victims and victims' families from reliving last summer's shootings, Recht says.